J Surfact Deterg (2018) 21: 53–63 DOI 10.1002/jsde.12007
ORIGINAL ARTICLE
Micellar Properties of Surface Active Ionic Liquid Lauryl Isoquinolinium Bromide and Anionic Polyelectrolyte Poly(Acrylic Acid Sodium Salt) in Aqueous Solution Amalendu Pal1 · Ritu Maan1
Received: 1 May 2017 / Revised: 19 September 2017 / Accepted: 9 October 2017 © 2018 AOCS
Abstract The complex formation between anionic polyelectrolyte poly(acrylic acid sodium salt) [NaPAA] and surface active ionic liquid (SAIL) lauryl isoquinolinium bromide [C12iQuin][Br] in aqueous media has been investigated by surface tension, isothermal titration calorimetry (ITC), and conductance. The self-assembled structures have been characterized using dynamic light scattering (DLS) and turbidity measurements. A range of surface parameters have been calculated from tensiometric measurements including critical micelle concentration (CMC), surface excess concentration (Γcmc), surface pressure at the interface (Πcmc), minimum area occupied at air–solvent interface (Amin), adsorption efficiency (pC20), and surface tension at the CMC (γ cmc). The thermodynamic parameters, i.e., standard enthalpy of micellization ΔHm , standard free energy of micellization (ΔGm ), and standard entropy of micellization (ΔSm ) have also been evaluated. Four different stages of transitions, corresponding to the progressive formation of NaPAA–[C12iQuin][Br] complex (C1), critical aggregation concentration (CAC), critical saturation concentration (C3) and CMC have been observed owing to strong electrostatic and hydrophobic interactions. The results obtained from DLS and turbidity measurements show that size of the aggregates first decreases and then increases in the presence of polyelectrolyte. The binding isotherms obtained using isothermal titration calorimetry (ITC) show the concentration dependence as well as the highly cooperative nature of interactions corresponding to formation of polyelectrolyte–SAIL complexes. * Amalendu Pal
[email protected] 1
Department of Chemistry, Kurukshetra University, Kurukshetra 136119, India
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Keywords Polyelectrolyte–SAIL complex Surface tension ITC Conductance DLS Turbidity J Surfact Deterg (2018) 21: 53–63.
Introduction An ionic liquid is a salt that exists as a liquid below 100 C. Ionic liquids existing as liquid at room temperature are termed as room-temperature ionic liquids (RTIL) (Aki, Brennecke, & Samanta, 2001). Their physicochemical properties are similar to high-temperature ionic liquids but their maintenance and handling aspects are quite distinctive. SAIL, often called ionic liquid (IL)-based surfactants, are amphiphilic molecules that possess the combined properties of both ionic liquids and surfactants (Tariq et al., 2012). Insight into their interfacial and aggregation properties provides possibilities for broadening future applications of IL. The aggregation behavior of IL in combination with different additives is of immense interest in fundamental and applied research areas like chemical synthesis, catalysis, electrochemistry, extraction, chromatography, synthesis of new materials, paints and coatings, cosmetics adhesives, and pharmaceutical products and many other applications (Brackmann & Engberts, 1993). Also, these complexes make a strong candidature for good electrolytic systems for high-performance capillary electrophoresis (Benedek & Thiede, 1994; Tseng, Lin, & Chang, 2002). The design of different SAIL structures has the potential to make an impact on the design and development of surfactants. The interactions between polyelectrolytes and surfactants are mainly hydrophobic interactions, electrostatic interactions,
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or hydrogen bonding. Hydrophobic interactions exist among the hydrophobic tails of polymer, as well as among surfactant molecules. These hydrophobic interactions may also exist among different hydrophobic tails of polymer and surfactant molecules. Electrostatic interactions exist between polymer and surfactant molecules and may be attractive or repulsive in nature depending upon the charges on them. Electrostatic interactions may also exist among the hydrophilic parts within the surfactant molecules. These interactions favor or disrupt the surfactant micellization process. Extensive studies have been carried out on polyelectrolytes and oppositely charged surfactants in aqueous solution. There is strong attraction between two oppositely charged species and the binding of surfactant monomers to polyelectrolyte chain starts at a very low surfactant concentration where even pure surfactant does not adsorb on the air–aqueous interface. Surfactant binding is highly cooperative in these systems, which is a consequence of contributions arising from interactions among the adsorbed SAIL molecules, and the formation of micelle-like clusters adsorbed on the polymer backbone. Recently, investigations on interactions between polymer chain and SAIL have received significant attention from researchers due to wide range of potential applications in the field of pharmaceuticals (Yoshida & Dubin, 1999), bioscience, water and soil treatment, oil recovery (Goddard & Ananthapadmanabhan, 1993; Kwak, 1998), and nanotechnology (Leonov et al., 2008). Therefore, the study of the effect of SAIL on aqueous systems comprising polymer is very broad. Recently, our group and Sharma, Kamal, Kang, and Mahajan (2015) examined the interaction of anionic polymer sodium polystyrene sulfonate (NaPSS) with cationic SAIL-based on imidazolium, [Cnmim][Cl] (n = 10,12,14) and [C8mim][Br] (Pal & Yadav, 2016). Zhang, Kang, Sun, Liu, and Wei (2013) have also studied the interactions of aqueous 1methyl-3-tetradecylimidazolium bromide [C14mim][Br] with NaPSS using various methods. It has been observed that the concentration of both surfactant and NaPSS has an effect on the formation of surfactant–polymer complex. The interactions between 1-dodecyl-3-methylimidazolium bromide and sodium carboxymethylcellulose (NaCMC) have also been studied by Liu, Zheng, Sun, and Wei (2010)). The aggregation behavior of pyridinium-based ionic liquids in aqueous solution has been discussed in detail (Bandres, Meler, Giner, Cea, & Lafuente, 2009; Cornellas et al., 2011; Jiang, Li, Yang, Cheng, & Yang, 2012; Sastry et al., 2012). Isoquinolinium compounds have applications in the medical field (Dabholkar & Tripathi, 2011) owing to their biologically active properties. They have been found to exhibit superior antimicrobial activity in comparison to similar analogues of pyridinium IL and quaternary ammoniumbased surfactants. Lauryl isoquinolinium bromide [C12iQuin]
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Scheme 1 Chemical structure of ionic liquid lauryl isoquinolinium bromide [C12iQuin][Br] and polyelectrolyte poly(acrylic acid sodium salt) (NaPAA)
[Br] is a solid cationic surfactant with a melting point of 49 C. The literature on isoquinolinium-based ionic liquids (Domanska et al., 2011; Domanska, Zawadzki, Królikowski, & Lewandrowska, 2012; Domanska, Zawadzki, & Lewandrowska, 2012; Domanska, Zawadzki, Paduszynski, & Królikowski, 2012; Lava, Evrard, Hecke, Meervel, & Binnemans, 2012; Visser, Holbrey, & Rogers, 2001) is very limited. The micellization of N-alkyisoquinolinium-based ionic liquids such as butyl isoquinolinium bromide [C4iQuin][Br], octyl isoquinolinium bromide [C8iQuin][Br], and lauryl isoquinolinium bromide [C12iQuin][Br] in aqueous solution have been reported by Zhang et al. ( 2014) using surface tension, electrical conductivity, and 1H nuclear magnetic resonance (NMR) measurements. The fact that short chain [n = 4 and 8] IL undergo micellization in aqueous media has been widely accepted. There is no report available on the aggregation behavior of IL [C12iQuin][Br] and NaPAA in aqueous media. The chemical structure of [C12iQuin][Br] and NaPAA is shown in Scheme 1. The aim of the current study is to describe the distinctive nature of thermodynamics of micellization of [C12iQuin][Br] and the micellar structure in aqueous solution of an oppositely charged polyelectrolyte poly(acrylic acid sodium salt) [NaPAA] using surface tension, isothermal titration calorimetry (ITC), electrical conductivity, dynamic light scattering (DLS), and turbidity measurements.
Experimental Materials 1-Bromododacane (>98%) and poly(acrylic acid sodium salt) (NaPAA) were purchased from Sigma-Aldrich, St. Louis, MO, USA. 1-Methylimidazole (99%) was purchased J Surfact Deterg (2018) 21: 53–63
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Table 1 The specification of chemicals Chemical name
1-Methylimidazole 1-Bromododecane Dichloromethane Poly(acrylic acid sodium salt)
CAS no.
Source
616-47-7 143-15-7 75-09-2 9003-04-7
Acros Organics, USA Sigma Aldrich, USA Sigma Aldrich, USA Sigma Aldrich, USA
Purity (mass%) ≥99.0 >98% – –
from Filderstadt, Germany which employed the Wilhelmy plate method. Successive additions of IL into the aqueous polyelectrolyte solution were made by volume. The solutions were stirred for a few minutes to achieve complete dissolution and allowed to equilibrate before each measurement. Three readings were noted and they were accurate within 0.02 mN m−1. A Julabo water thermostat was used in order to control temperature with an uncertainty of 0.01 K. Isothermal Titration Calorimetry
from Acros Organics. Methanol (99%) was bought from Rankem, Haryana, India. All aqueous solutions have been prepared using Millipore grade water. The specific conductivity and surface tension of Millipore grade water were measured and found to be 3 μS cm−1 and 71 mNm−1, respectively. The ionic liquid, lauryl isoquinolinium bromide [C12iQuin][Br] was synthesized in our laboratory. 1H NMR technique was used to verify the purity of [C12iQuin] [Br]. The detailed information of chemicals used in the present study is provided in Table 1. Synthesis of Lauryl Isoquinolinium Bromide [C12iQuin][Br] Isoquinoline and alkyl bromide were weighed into a 250-mL round-bottomed flask and dissolved in acetonitrile under a nitrogen atmosphere. The reaction mixture was heated to reflux for 2 h. Dichloromethane was added to the crude product after refluxing followed by activated carbon to achieve decolorization. Finally, the red-colored, viscous oil was allowed to cool to room temperature. Several washings with n-hexane were performed in order to achieve recrystallization and remove any unreacted reagents. [C12iQuin][Br] was kept under vacuum for 3 days to ensure the elimination of any solvent residue. The moisture content was found to be less than 0.02 wt % by Karl-Fischer titration. The structure of [C12iQuin][Br] was confirmed by analyzing its 1H NMR spectra. The NMR details of corresponding protons for [C12iQuin][Br] are given below: DMSO-d6 0.85 (t,3H), 1.22(m,20H),4.76(t,2H), 8.28(t,1H), 8.40(t,1H), 8.53(d,1H),8.65(d,1H),8.89(d,1H) 10.23(S,1H).
Calorimetric titrations of samples were performed using a MicroCal ITC200 microcalorimeter. Millipore water was filtered into the sample cell and the reference cell and maintained at 298.15 K. A Hamiltonian syringe that was automatically controlled by the instrument was filled with approximately 40 μL of IL solution in water and aqueous polyelectrolyte solution. The surfactant (approximately 2 μL) was added to the sample cell having water or aqueous polyelectrolyte solutions (approximately 200 μL) and stirred continuously at a speed of 500 rpm. The instrument is equipped with software to control parameters such as time and duration between each successive addition. Each addition was accompanied with change in enthalpy and this change at different concentrations was plotted with the help of instruments’ origin software. Conductivity Measurements A digital conductivity meter (CM-183) with microprocessorbased EC-TDS analyzer with ATC probe was used to measure the electrical conductivities of the solution. The conductivity cell with electrodes made up of platinum and a cell constant of 1.003 cm−1 was purchased from Elico Ltd., Hyderabad, India. It was calibrated with aqueous KCl solutions (0.01–1.0 mol kg−1) before any experimental measurement was made. The measurements were performed in a water-jacketed, double-walled flow dilution cell with an uncertainty of 0.01 K at 298.15 K. Three readings were noted for every particular concentration and mean values were reported. The uncertainty in the electrical conductivity measurements was less than 4%.
Instruments and Methods Dynamic Light Scattering and Turbiditimetry Aqueous solutions were prepared using Millipore grade water. A&D Company Limited electronic balance (Tokyo, Japan, model GR-202) with an accuracy of 0.01 mg was used to weigh the chemicals. Tensiometry Surface tension measurements were performed at 298.15 K using a Data Physics, Model DCAT-II automated tensiometer J Surfact Deterg (2018) 21: 53–63
DLS measurements were carried out with the help of NaBiTecSpectro Size300 light-scattering apparatus (NaBiTec, Lüneburg, Germany) with a He−Ne laser source (633 nm, 4 mW) at 298.15 K. A 0.45-μm membrane filter was used to filter [C12iQuin][Br]–NaPAA solutions of appropriate concentrations into the quartz cell which was previously rinsed with filtered water. The measurements were performed at controlled temperature within accuracy of 0.1 K.
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55 0 g/L NaPAA
0.005 g/L NaPAA
50
50
45
45
40
C1
40 C2
35
γ (mN m-1)
C3
cmc
cmc
35
30 0.37
1.00
2.72
7.39
0.02
0.05
0.14
0.37
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55 0.01 g/L NaPAA
30 7.39 55
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40 C2 C3
cmc
2.72
cmc 35
7.39
0.05
0.14
0.37
1.00
2.72
30 7.39
ln [C12 iQuin][Br] Fig. 1 Surface tension of aqueous solutions of [C12iQuin][Br] and NaPAA as a function of added [C12iQuin][Br] in the absence ( ) 0 g L−1, and presence of different concentrations ( ) 0.005 g L−1, ( ) 0.010 g L−1, ( ) 0.025 g L−1 of NaPAA at temperature 298.15 K
The laser light from the source falls on the cell containing the sample and the light scattering signal was detected at 90 . The instrument had inbuilt CONTIN algorithm which was used to evaluate the DLS data. Turbiditimetric measurements of the solutions were performed using (Eutech TN-100, Mumbai, India) turbiditimeter. [C12iQuin][Br]–NaPAA solutions of appropriate concentrations were prepared followed by stirring for few minutes. Then the samples were allowed to equilibrate for approximately 5 min and the measurements were performed.
Results and Discussion The complex formation between [C12iQuin][Br] and NaPAA in aqueous media at various concentrations of NaPAA was studied using different techniques such as surface tension, ITC, conductance, DLS, and turbiditimetry. Tensiometry In systems containing polyelectrolyte and surfactant of opposite electrical charge, there is an uneven distribution of
ions in solution due to electrostatic interactions leading to enhanced concentration of surfactant ions in the proximity of the polyelectrolyte. As the distance from the polyelectrolyte increases, the concentration of surfactant ions decreases. Cooperative binding of SAIL molecules to an oppositely charged polyelectrolyte is a highly favorable phenomenon due to electrostatic interactions. The relative concentration of SAIL molecules at the interface and in the bulk determines the variation in surface tension. Tensiometric profiles of [C12iQuin][Br] in different concentrations of aqueous NaPAA (0, 0.005, 0.010, and 0.025 g L−1 NaPAA) are plotted in Fig. 1. For pure [C12iQuin][Br], the surface tension gradually decreases with increasing SAIL concentration until a knap regime, beyond which surface tension does not decrease further. However, in the presence of polyelectrolytes, many different transitions have been observed. C1, C2, C3, and CMC are distinct characteristic concentrations where the surface tension changes quite sharply, and are generally observed in surfactant-polyelectrolyte mixtures (Bell, Breward, Howell, Penfold, & Thomas, 2010; Bharmoria & Kumar, 2013; Staples, Tucker, Penfold, Warren, & Thomas, 2002). With the addition of SAIL, the surface tension decreases at C1 due to the progressive formation of the NaPAA– [C12iQuin][Br] complex. At higher SAIL concentration, J Surfact Deterg (2018) 21: 53–63
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Table 2 The various characteristic interaction concentrations C1, C2, C3, and CMC (mmol L−1) observed from surface tension, isothermal titration calorimetry and conductance measurements of [C12iQuin] [Br] in the absence and presence of the polyelectrolyte NaPAA at 298.15 K [NaPAA] (g L−1)
0.000 0.005
0.010
0.025
Surface tension
ITC
Conductance
5.07 0.24 1.49 2.97 3.83 0.19 0.87 1.81 3.78 0.25 2.01 3.45 4.45
5.15 – – – 5.09 – – – 4.92 – – – 4.98
5.73 – – 4.36 5.59 – – 4.33 5.23 – – 4.41 5.43
CMC C1 C2 C3 CMC C1 C2 C3 CMC C1 C2 C3 CMC
Uncertainties are (conc. of NaPAA) = 1 × 10−3 g L−1, s(cmc)S. −5 −1 −5 (mol L−1), T. = 2 × 10 (mol L ), s(cmc)ITC = 1 × 10 s(cmc)cond. = 1 × 10−5(mol L−1), s(T) = 1 × 10−2 K,s (p) = 2 kPa.
abrupt increase that is also observed by other experimental techniques. This interesting phenomenon can be accounted for overriding of hydrophobic interactions over electrostatic attractions between oppositely charged SAIL and polyelectrolyte due to which micellization takes place at relatively higher concentration of polyelectrolyte. Using the surface tension data, various surface parameters such as surface excess concentration (Γmax), surface pressure at the interface (π cmc), and minimum area at air– solvent interface (Amin) covered by single monomer of SAIL and surface tension at the CMC (γ cmc) have been determined using the Gibbs equation (Dong et al., 2008) and their corresponding values are given in Table 3. The surface excess concentration (Γmax) was calculated using the following equation (Moroi, 1992): dγ = −nRTΓmax d lnCT
where γ is surface tension, R is universal gas constant, T is temperature, n represents number of ions formed in solution, and C denotes SAIL’s concentration. The minimum area occupied by the SAIL, Amin has been calculated from (Pal & Yadav, 2015): Amin =
the surface tension shows an inflection point, C2, called the critical aggregation concentration (CAC) due to the formation of surface-active NaPAA–[C12iQuin][Br] aggregates. Beyond the CAC, a cooperative binding of NaPAA with SAIL molecules is observed due to hydrophobic and electrostatic interactions, resulting in precipitation, which is a commonly observed phenomenon in oppositely charged polyelectrolyte–SAIL systems (Staples et al., 2002). At even higher SAIL concentrations, micelles are formed at the CMC after which the surface tension remains more or less constant. At C1, the addition of polyelectrolyte has been found to decrease the surface tension due to the synergistic effect resulting from strong attractive interactions between polyelectrolyte and the head groups of oppositely charged SAIL molecules (Goddard, 2002). The effect is significant at very low surfactant concentrations where even pure surfactant is incapable of adsorbing at the surface of the solution. In this SAIL concentration regime, the surfactant molecules appear to be noncooperatively bound to the polymer backbone. All these characteristic interaction concentrations have been tabulated in Table 2. From Fig. 1 and Table 2, it is observed that the CMC decreases with the addition of 0.005 g L−1 NaPAA solution. The CMC decreases further upon increasing the concentration of polyelectrolyte from 0.005 to 0.01 g L−1 NaPAA. However, a different trend is observed as we increase the concentration further from 0.01 to 0.025 g L−1 NaPAA. The CMC value takes an J Surfact Deterg (2018) 21: 53–63
ð1Þ
1020 NA •Γmax
ð2Þ
where NA is Avogadro’s number. The values of Γmax and Amin have been found to follow the opposite trend. For [C12iQuin][Br]–NaPAA system in aqueous solution of different concentrations of NaPAA, Γmax decreases and Amin increases (with an exception of 0.010 g L−1 NaPAA) signifying that there exists higher concentration of SAIL molecules at the air–aqueous interface. The increasing values of Amin indicate that packing density increases with polyelectrolyte concentration at the air–liquid interface. pC20 is an important surface parameter, which is known as the adsorption efficiency and can be obtained from the equation: Table 3 The surface parameters, i.e., surface tension at cmc (γ cmc), effective surface tension reduction (π cmc), surface excess (Γmax), minimum area per molecule (Amin), and adsoption efficiency (pC20) of [C12iQuin][Br] and [C12iQuin][Br] at different NaPAA concentrations [NaPAA] (g L−1)
γ cmc (mN m−1)
T (298.15 K) 0.000 30.30 0.005 32.46 0.010 32.38 0.025 33.59
π cmc (mN m−1)
Γmax × 106 (mol m−2)
Amin (nm2)
pC20
39.72 27.55 25.62 31.41
1.36 0.20 0.16 0.17
1.22 8.21 9.79 9.50
0.03 1.35 1.36 1.20
Uncertainties are s(conc. of NaPAA) = 1 × 10−3 g L−1, s(π cmc) = 0.2 (mN m−1), s(Γmax) = 0.03 × 10−6 (molm−2), s(Amin) = 0.02 (nm2), s(T) = 1 × 10−2 K, s(p) = 2 kPa.
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J Surfact Deterg 0 g/L NaPAA
0.005 g/L NaPAA
1600
1600
1200
1200
ΔH m o
ΔHom
ΔHd (kJ mol-1)
800
800
400 0
400 0
2
4
6
8
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12
0
2
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6
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0.025 g/L NaPAA
0.01 g/L NaPAA
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1600 1200
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ΔH m o
800
800
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400 0
2
4
6
8
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0
2
4
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-1
[C12iQuin][Br] (mmol L ) Fig. 2 Enthalpograms of [C12iQuin][Br] in the absence ( ) 0%, and presence of different concentrations ( ) 0.005 g L−1, ( ) 0.010 g L−1, ( ) 0.025 g L−1 of NaPAA at temperature 298.15 K
pC 20 = − log C20
ð3Þ
where C20 is the SAIL’s concentration that decreases the value of γ for pure water by 20 mN m─1. This is the lowest concentration which is required for saturation of surface adsorption. The values of pC20 have been found to increase with increase in the concentration of NaPAA from 0.005 g L−1 NaPAA to 0.01 g L−1 NaPAA for [C12iQuin] [Br]–NaPAA system which indicates that the adsorption efficiency has increased at the air–aqueous interface upon increasing the concentration of NaPAA. Although upon increasing the concentration of NaPAA further from 0.01 g L−1 NaPAA to 0.025 g L−1 NaPAA, there is a substantial decrease in the pC20 values which indicates that the adsorption efficiency has decreased with increase in NaPAA concentration. It suggests that at higher concentration of NaPAA, there is a decrease in the surface activity of SAIL. Further, the parameter π cmc, the comparative effectiveness of the SAIL molecule, has been estimated by surface pressure at the CMC using the following equation (Pal & Yadav, 2015): π cmc = γ 0 − γ cmc
ð4Þ
where γ cmc is the surface tension at the CMC for a solution of particular concentration and γ 0 is the surface tension of pure solvent. The values of π cmc have been found to
decrease with an increase in the concentration of NaPAA which implies that the efficiency of SAIL in reducing the value of surface tension decreases in the presence of polyelectrolyte. However, it increases at 0.025 g L−1 NaPAA. In the presence of NaPAA, same trend is seen for both γ cmc and π cmc, which indicates that SAIL’s efficiency in reducing the surface tension decreased with increasing polyelectrolyte concentration.
Isothermal Titration Calorimetry In order to better understand the interactions between SAIL and polyelectrolyte, the isothermal titration microcalorimetry (ITC) technique has proved very useful (Bai, Nichifor, Lopes, & Bastos, 2005) and different parameters like enthalpy of micellization (ΔHom) and CMC can be determined. Each surfactant has a characteristic concentration at which micellization occurs and the effect of cosolute can be very striking. In the quest to understand the possible underlying interactions among SAIL–polyelectrolyte system, the calorimetric titration curves of [C12iQuin][Br] and NaPAA aqueous solutions are shown in Fig. 2. The existence of endothermic peaks (positive values of enthalpies of dilution, ΔHd) in the ITC curves suggests that the interaction between the SAIL and polymer is controlled by polymer-induced micellization. Also, these effects are due J Surfact Deterg (2018) 21: 53–63
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Table 4 The thermodynamic parameters for the binding and micellization of [C12iQuin][Br] at different NaPAA concentrations
[NaPAA] (g L−1)
CMC (Cond.) (mmol L−1)
α(Cond.)
ΔGm (kJ mol−1)
ΔHm (ITC) (kJ mol−1)
TΔSm (kJ mol−1)
T (298.15 K) 0.000 0.005 0.010 0.025
5.73 5.59 5.23 5.43
0.62 0.58 0.45 0.54
−31.67 −33.65 −35.77 −34.07
−6.39 −5.97 −5.84 −5.88
25.28 27.68 29.94 28.20
Uncertainties are s(conc. of NaPAA) = 1 × 10−3 g L−1, s(cmc)cond. = 1 × 10−5(mol L−1), s(ΔGm ) = 0.02 (kJ mol−1), s(ΔHm ) = 0.01 (kJ mol−1), s(ΔSm ) = 0.02 (kJ mol−1), s(T) = 1 × 10−2 K, s(p) = 2 kPa.
to dilution of concentrated micelles of surfactant when they are titrated to aqueous polyelectrolyte solutions. All the dilution enthalpy curves have sigmoidal shape and the CMC can be obtained from the center point of the curve. The plateau stays flat after the CMC in these enthalpograms. The standard enthalpy of micellization (ΔHom) can be determined by the difference in the heat change between the first and last knap sections at the position of discontinuity. The observed enthalpy changes as a function of [C12iQuin][Br] concentration have been plotted in Fig. 2. The enthalpies of micellization for [C12iQuin][Br] in water as well as in different polyelectrolyte concentrations are exothermic. It becomes less exothermic when the concentration of NaPAA increases (Table 4). This suggests that ΔHom depends on the hydrophobic interaction between SAIL and polyelectrolyte. However, the addition of SAIL to the polyelectrolyte solutions of all the desired concentrations has been found to be an endothermic process for which electrostatic interaction among the SAIL and polyelectrolyte chains leading to monomeric adsorption can be held accountable. In the absence of NaPAA, a gradual decrease is seen in ΔHd values from the endothermic region towards relatively less endothermic regions, leading to an overall exothermic effect. At lower concentration of NaPAA, i.e., before the CMC, the hydrophobic region of [C12iQuin][Br] expands and monomers of [C12iQuin] [Br] exposed to more water interact with the NaPAA backbone and start forming aggregates that appear as an exothermic phenomenon. The process continues till the minimum point of curve starts. Further addition of SAIL results in formation of premicellar aggregates mainly because of electrostatic repulsion between similarly charged head groups. [C12iQuin]+ and [PAA]+ dominate over the electrostatic repulsion between IL head groups and counter-ions, which leads to an increase in magnitude of endothermic enthalpy changes for micelle formation of IL, i.e., at 0.025 g L−1 when similar change has been observed, but smaller in magnitude. It is expected that electrostatic repulsions between the charged groups are more prominent J Surfact Deterg (2018) 21: 53–63
than its interactions with monomers of SAIL present at lower concentration, which leads to an insertion of [C12iQuin]+ into the already present micelles. It has been observed from the surface tension measurements that the CMC values for the SAIL–polyelectrolyte aqueous solutions are lower than the CMC of pure aqueous solutions of SAIL. At the lower concentrations, the CMC decreases but when the concentration becomes appreciably higher, a slight increase has been noticed as observed from the tensiometric measurements. The CMC values obtained from ITC have been found to be higher than CMC values obtained from the tensiometric measurements. This is due to slow process of micellization of SAIL–polyelectrolyte system because in tensiometric measurements, CMC is considered as the point where no more surfactant molecule can be adsorbed at the air–water interface while in isothemal titration calorimetry, the CMC corresponds to onset of micelle formation. Here, we have also observed that the CMC first decreases upon increasing the concentration to 0.01 g L−1 NaPAA. Later, the CMC again decreases upon increasing the concentration of NaPAA to 0.025 g L−1 NaPAA (Table 2). The values of enthalpies of micellization (ΔHom) obtained here have been further used to calculate various thermodynamic parameters of micellization in the next section. Conductance Measurements The experimental electrical conductivities at temperature 298.15 K for [C12iQuin][Br] in aqueous solutions of NaPAA in different concentrations (0, 0.005, 0.01, and 0.025 g L−1 NaPAA) are shown in Fig. 3 as a function of SAIL concentration. The values of CMC and various parameters are listed in Table 2. In the electrical conductivity versus concentration plots, two straight lines have been obtained. The CMC value has been assigned to the point of intersection of these two tangential lines. The conductometric profiles of the systems studied show a decrease in the values of the CMC of [C12iQuin][Br] after the addition of polyelectrolyte NaPAA. This indicates that the attractive
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300
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0 0
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8
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-1
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8
[C12iQuin][Br] (mmol L ) Fig. 3 Specific conductance (κ) of aqueous [C12iQuin][Br] solutions in the absence ( ) 0%, and presence of different concentrations ( ) 0.005 g L−1, ( ) 0.010 g L−1, ( ) 0.025 g L−1 of NaPAA at temperature 298.15 K
electrostatic interaction between the charged head groups of SAIL and polyelectrolyte is dominant over the hydrophobic interactions existing among the hydrophobic tails of both the interacting components. These electrostatic interactions facilitate early micelle formation. From Fig. 1 it is clear that the CMC has initially decreased upon addition of surfactant to lower concentrations of NaPAA solution because the phenomena of micelle formation has taken place. The repulsion among head groups reduces at the micellar surface because of electrostatic interactions and the CMC has been reduced to a much lower concentration than in the case of a pure SAIL system. However, the CMC has increased in higher concentrations of polyelectrolyte because the hydrophobic interactions come into play and stabilize individual surfactant’s monomers delaying the micellization process. The ratio of slopes of pre-and postmiceller region in the conductance versus concentration profile (Inoue, Ebina, Dong, & Zheng, 2007; Singh & Kumar, 2008; Wang, Wang, Zhang, Zhang, & Zhao, 2007; Wang, Zhang, Wang, & Wu, 2011) provides the degree of counter-ion dissociation (α). In the conductivity profiles, a break has been observed in the high-concentration region of SAIL molecules which is around C2 in the surface tension trend followed by the CMC. The redissolution leading to restricted movement of counter-ions can be held accountable for the slow increase in conductance post CMC. Finally, on further
addition of SAIL, more and more SAIL molecules adsorb on the backbone of polyelectrolyte, leading to enhanced positive charge on the polyelectrolyte. If the concentration of polyelectrolyte is increased, values of CMC of systems under investigation decrease except at appreciable higher concentration for the reason mentioned in the previous section. Various thermodynamic parameters of micellization have been calculated for the SAIL– polyelectrolyte system of current interest using the temperature dependence of the CMC. The standard Gibbs free energy (ΔGmo ) of micellization for the SAIL–polyelectrolyte system has been calculated using the equation (Rosen, 1988): ΔGm = ð2− αÞRT lnXcmc
ð5Þ
where Xcmc is the CMC that has been obtained from the electrical conductivity versus concentration plots and has been expressed in the mole fraction. The entropy of micellization, (ΔS m) has been calculated using ΔH mvalues obtained using the ITC curve from the following equation (Rosen, 1988). ΔSm =
ΔH m − ΔGm T
ð6Þ
The thermodynamic variables have been calculated and displayed in Table 4. Changes in different parameters have been observed with change in polyelectrolyte concentration, J Surfact Deterg (2018) 21: 53–63
61
0.8 0.0 0.8 0.0 0.8 0.0 0.8
60
5.002 mM 3.775 mM
0.005 g/L NaPAA 0.01 g/L NaPAA 0.025 g/L NaPAA
50
3.052 mM 0.694 mM
0.0 0.8 0.0 0.8 0.0 0.8
0.183 mM 0.154 mM 0.122 mM
0.0 0
20
40
60
80
100
120
140
160
180
200
Rh(nm)
Turbidity (NTU)
Intensity (%)
J Surfact Deterg
40 30 20 10 0
Fig. 4 Distribution of the hydrodynamic diameter Rh for the 0.010 g L−1 NaPAA solution in the presence of different [C12iQuin] [Br] concentrations at temperature 298.15 K
and they collectively point towards some important phenomenon occurring in SAIL–polyelectrolyte aqueous solutions. The values of TΔS m have been found to be positive while ΔH m being negative for all the concentrations of NaPAA, indicating that micellization process has been codriven by entropy and enthalpy. The micellization process is exothermic. The aliphatic chains of SAIL molecules have gone under hydrophobic salvation because of enclosing SAIL molecules in between the much-ordered structure of water molecules as compared with free water molecules. It distorts the hydrophobic hydration through micelle formation followed by higher entropic state of the system as a whole. It contributes to the formation of micelle (Asker, Weiss, & Mcclements, 2009; Bijma & Engberts, 1994). With the increase in polyelectrolyte concentration, ΔH m decreases, indicating that less heat has been evolved with increase in polyelectrolyte concentration. Also, it is inferred that the enthalpy change contribution is lesser than the entropy-driven phenomenon during micellization process at all concentrations. The values of ΔG m become negative upon increasing the concentration of NaPAA, indicating that the process has become more spontaneous. The efficiency of SAIL has been improved in the presence of polyelectrolyte. Dynamic Light Scattering and Turbiditimetry DLS measurements were performed to determine the hydrodynamic size, geometrical shape, and type of selfassembled aggregates of [C12iQuin][Br] in aqueous medium in the presence of 0.01 g L−1 NaPAA solution to identify how the size distribution varies with change in concentration. The hydrodynamic radii, Rh, of the selfassembled structures measured from DLS are shown in Fig. 4. Upon the addition of SAIL in the concentration range below C1, SAIL monomers start to bind to the chains J Surfact Deterg (2018) 21: 53–63
0
5
10
15
20
25
-1
[C12iQuin][Br] (mmol L ) Fig. 5 Turbiditimetric profiles of [C12iQuin][Br] in the presence of different concentrations ( ) 0.005 g L−1, ( ) 0.010 g L−1, ( ) 0.025 g L−1 of NaPAA at temperature 298.15 K
of polymer because of electrostatic attraction as observed in both the electrical conductivities and tensiometric measurements. At C1, two different peaks are witnessed in DLS measurements. The two different peaks show the existence of both the polyelectrolyte monomer and multimers. Initially the second peak is small and narrow, which shows that the formation of multimers has just started. Later the second peak becomes broader showing the existence of polyelectrolyte and SAIL complex. The first peak with smaller hydrodynamic radii corresponds to soluble complex formed between SAIL and polyelectrolyte in the diluted equilibrium solution while the latter with larger hydrodynamic radii has been allocated to the insoluble aggregates of SAIL and polyelectrolyte. If the surfactant addition is continued further, there is a decrease in Rh, leading to the coiling up of structure of NaPAA–[C12iQuin][Br] complex, as a result of which the hydrophobic parts of the SAIL approach each other. This creates aggregates of bounded SAIL molecules to the polyelectrolyte chain. Finally, for [C12iQuin][Br], at a concentration exceeding the CMC, hydrodynamic radii shrink due to existing polyelectrolyte– SAIL complex. An attempt has also been made to gain insight into the size of complexes formed among [C12iQuin][Br] and NaPAA in the concentration range of interest using turbiditimetry. The change in turbidity of SAIL in different concentrations of aqueous polyelectrolyte solutions is shown in Fig. 5. The turbidity increases with increase in SAIL concentration until precipitation is observed. At surfactant concentrations exceeding the CAC, the turbidity sharply decreases and becomes roughly constant due to redissolution of precipitates.
62
Conclusion Studies related to the micellization behavior of a cationic surfactant [C12iQuin][Br] in aqueous solutions of an oppositely charged polyelectrolyte NaPAA have been performed using surface tension, ITC, conductance, DLS, and turbidity measurements. The CMC values obtained from the different techniques have been found to be correlated well with each other. With the progressive addition of SAIL to the aqueous solutions of polyelectrolyte, various transitions have been observed. Initially, the C12iQuin][Br]–NaPAA complex is formed due to attractive electrostatic interactions followed by precipitation and finally micelle formation after which the surface tension does not decrease further. The CMC decreases upon increasing the concentration to 0.01 g L−1 NaPAA but shows an abrupt increase upon increasing the concentration further to 0.025 g L−1 NaPAA, showing the dominance of hydrophobic interactions among SAIL and polyelectrolyte over Coulombic attractions. The addition of SAIL to polyelectrolyte is an endothermic process at all concentrations of the polyelectrolyte. However, ΔH m is exothermic. Further, from the conductometric measurements, negative values of ΔGm and positive values of ΔSm show that the micellization process is spontaneous and entropy driven. The self-assembled structures have been characterized with DLS and turbidity. The hydrodynamic radii start increasing due to binding of SAIL to polyelectrolyte chains followed by two different peaks related to monomers as well as multimers and finally Rh shrinks due to existence of SAIL–polyelectrolyte complex. The turbidity initially decreases after C2 due to precipitation and then becomes constant because of redissolution of precipitates. Acknowledgements This work is supported by the Council of Scientific and Industrial Research, Government of India (Grant No. 21(1005)/15/EMR-II) through Emeritus Scientist grant to Prof. A. Pal. The authors thank Praveen Singh Gehlot, Research Scholar at CSIR-CSMCRI, Bhavnagar, for assisting in experimental measurements.
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Biographies Amalendu Pal is an emeritus scientist (CSIR) in the department of chemistry, kurukshetra University, kurukshetra, India. He obtained his Ph.D. in chemistry from Kalyani University, Kalyani, India in 1984. His studies involve physical chemistry of solutions, experimental and theoretical thermodynamic study of solutions and colloid and interfaces. Ritu Maan is a research associate in the department of chemistry, kurukshetra University, kurukshetra, India. She obtained her Ph.D. in chemistry from Maharishi Markandeshwar University, Haryana, India in 2015. Her studies involve the thermodynamics of solutions and colloidal and interfacial properties of surface active ionic liquids.
